Amplifiers having a gain characteristic which is exponential have a voltage transfer function which may be expressed as: EQU V.sub.out =A log V.sub.in /V.sub.ref,
where A is a gain factor, V.sub.in is an input voltage and V.sub.ref is a reference voltage. Diode-based circuits, which inherently possess this transfer function, are susceptible to temperature variations and are also subject to a logarithmic reduction in frequency response with decreases in signal level. In recent years circuits have been developed which can provide a logarithmic output over a respectable dynamic range of input signal levels. A data sheet published by Analog Devices for the AD600 variable gain amplifier mentions that the circuit can handle inputs from 100 .mu.V to 1 V rms with a constant measurement bandwidth of 20 Hz to 2 MHz and a logarithmic transfer function. Improved bandwidth characteristics are obtained by using a R-2R resistor network as the input to a difference amplifier. This is more fully described in U.S. Pat. No. 5,077,541 to B. Gilbert. One of the main advantages of using the R-2R resistor network is that the effect of temperature variations are greatly reduced. Moreover, frequency response is relatively constant over a far greater bandwidth than can be achieved with diode-based circuitry.
It has been previously recognized that R-2R wideband amplifiers can be cascaded and achieve very high gain linearity over a 100 dB range.
One of the problems introduced in cascading stages is that there are uncompensated offsets between stages which can affect linearity and dynamic range of the output. The offsets vary with gain and so prior cascaded circuits usually use AC coupling. A problem with AC coupling is that the amplifiers cannot operate at low or zero frequencies.
An object of the invention was to devise a wideband logarithmic amplifier having circuitry for reducing the effect of offset voltages at low frequency and DC levels.